Plan year 

 a year equaling twelve consecutive months.

A plan year is permitted to begin on any day of a year and must end on the preceding day in the immediately following year (for example, a plan year that begins on April 5, 2011, must end on April 4, 2012). Once established, a plan year is effective for the first plan year and for all subsequent plan years, unless changed, provided that such change will only be recognized if made for a valid business purposes. A change in the plan year is not permitted if a principal purpose of the change in plan year is to circumvent the rules of section 4980H or these regulations.